蒸發液膜與層流氣流熱質傳現象之研究

Abstract

A combined theoretical and experimental study was conducted to investigated the evaporative cooling process of a free falling liquid film evaporating into air stream. Attention was particularly focused on the latent heat transfer associated with the film vaporization. In the first place, a numerical analysis was carried out to study the heat transfer characteristics for a liquid ethanol film falling down along an insulated vertical plate. The conservation equations for the liquid film and the induced gas flow were solved simultaneously. Meanwhile an experimental system was set up to measure the overall cooling of the film. The measured data are in good agreement with the numerical predictions. The detailed heat and mass transfer processes in laminar gas stream forced flowing over a liquid water film were also examined Significant liquid cooling results for the system with a high inlet liquid temperature, high gas stream velocity or a low liquid flowrate. Next, heat transfer enhancement through liquid film vaporization into mixed convection gas stream were investigated. The wetted plate is subjected to an uniform heat flux qw". Results show that the evaporative latent heat transfer depends largely on the inlet liquid temperature TL,in and wall heat flux qw" Additionally, the Nusselt number NuL were correlated. It is noted that heat transfer between the liquid film and gas stream is dominated by the transport of latent heat in conjuction with the evaporation of the liquid film.